The present invention relates to aqueous, radiation-curable binder dispersions which are free from solvents. The dispersions contain a radiation-curable binder and a radiation-curable emulsifier.
(Meth)acryloyl-containing polyurethanes (so-called urethane (meth)acrylates) and polyepoxides have been known for a long time and are very highly suited as binders, for example, for UV-curable coating materials (cf. e.g., DE-A 27 37 406). The relatively high viscosity of these products almost always makes it necessary to use them in conjunction with organic solvents.
Water-dilutable urethane (meth)acrylates also are known (cf. e.g., DE-A 29 36 039). The hydrophilicity of these known systems is based on the presence of ionic centers, especially sulfonate groups, which contain as counterions alkali metal cations or ammonium ions (from tertiary amines), at least some of which remain in the coatings ultimately obtained from the systems, to the considerable detriment of the water resistance of said coatings.
Another possible way to obtain water-dilutable products is to use external emulsifiers. For example, in accordance with U.S. Pat. No. 4,070,323, polyurethanes that contain acryloyl groups are dispersed in water using anionic or cationic oil-in-water emulsifiers, for example, sodium lauryl sulfate. In the course of free-radical crosslinking these emulsifiers are not incorporated into the coating film. The result of this is a substantial reduction in the degree of water resistance which can be achieved in the coating films.
DE-A 39 00 257 describes how, by reacting a polyethylene glycol with a polyisocyanate and a hydroxyalkyl (meth)acrylate, a hydrophilic, radiation-curable urethane acrylate is synthesized which can be used as an emulsifier for hydrophobic urethane acrylates. The emulsifying properties of these products are low and the stability of the dispersions prepared from them is limited.